The future of the Assembly was plunged into doubt last night after Sinn Fein dramatically pulled out of talks.

The parties now look set to miss this afternoon's deadline for a deal, signalling a period of political limbo with neither devolution nor direct rule from Westminster in place, and civil servants left in charge.

Observers warned that such unprecedented uncertainty could be disastrous, with no budget agreed and Brexit due to be triggered on Wednesday.

The parties have until 4pm today to reach an agreement and form an Executive but, in a surprise move yesterday afternoon, Sinn Fein declared the current phase of talks had "run its course" and it would not be nominating a Deputy First Minister.

Sinn Fein's Stormont leader Michelle O'Neill said the party had "come to the end of the road" regarding the current negotiations. "The talks process has run its course and Sinn Fein will not be nominating for the position of Speaker or for The Executive Office," she commented.

Mrs O'Neill accused the British Government and DUP of failing to "step up to the plate" on equality and human rights issues.

Sinn Fein president Gerry Adams said: "The DUP's approach thus far has been to engage in a minimalist way on all the key issues including legacy, an Irish Language Act, a Bill of Rights and marriage equality.

"They have been reinforced in this by the British Government's stance. This is unacceptable and a matter of grave concern."

However, Mr Adams insisted his party remained committed to power-sharing. "Will we be back, will we get the institutions in place? Yes," he said.

DUP leader Arlene Foster accused Sinn Fein of being rigid and uncompromising. "Throughout the course of Saturday Sinn Fein behaved as if they were the only participants whose mandate mattered. This cannot and will not be the basis for a successful outcome," she said.

"Negotiations will only ever be successful when parties are prepared to be flexible in order to secure outcomes. To date, there was little to suggest that Sinn Fein want to secure agreement."

Mrs Foster said her party had been willing to form a new administration without preconditions. "The DUP stands ready to continue to discuss how we can secure new arrangements for Northern Ireland," she stated.

As Sinn Fein and the DUP engaged in a war of words over who was to blame for the political crisis, the focus shifted to Secretary of State James Brokenshire.

He must decide whether to call another snap election, reintroduce a form of direct rule, or opt for a fudge to give the parties more time to reach agreement.

With Parliament due to rise for the Easter recess on Thursday, political sources predicted that he wouldn't act for at least another three weeks when they expected he would introduce emergency legislation to allow for a limited return to 'soft' direct rule.

But talks sources held out no hope, and revealed that an Irish Language Act had been the main stumbling block to progress.

"The DUP wanted a wider, watered-down Act that made provision for Ulster-Scots. That wasn't acceptable to Sinn Fein or the SDLP," an insider said.

"The problem has been that while Sinn Fein has a series of demands, the DUP are quite content with the status quo and don't have any big asks. There couldn't be any horse-trading because there weren't any horses for trade with the DUP."

Sources said the two Governments were "deeply frustrated" at both parties' attitudes. "The DUP didn't turn up yesterday because it refused to negotiate on a Sunday and, while Sinn Fein did show, it went home in a huff early," said a Stormont insider.

SDLP leader Colum Eastwood blamed the DUP for the talks' failure. "Rigid opposition to compromise on key issues, particularly from the DUP, has made a comprehensive resolution more difficult to reach," he said.

He claimed the DUP hadn't "got the message" that people were fed up with the lack of "equality and respect". He urged Mr Brokenshire "to create space immediately for a reconvened and refreshed process to allow for the restoration of power-sharing".

UUP chief negotiator Tom Elliott warned that unless there was "a massive U-turn" in attitude from the DUP and Sinn Fein, Northern Ireland faced "a period of prolonged drift".

He claimed that the current talks process had been the worst set of negotiations his party had ever been involved in.

"It was a clear indication that the bigger parties are even more dismissive of the smaller ones than ever, and demonstrates their inability to move away from their old ways," he added.

Alliance leader Naomi Long warned that the consequences of the talks' failure were grave and urged all parties to "step up and get real". She said: "For the first time in the 96-year history of Northern Ireland, we will be without any legal, political authority."